A phylogeny of the 22 species currently recognized in the genus
Megalagrion, endemic to the Hawaiian Islands, is presented based on an analysis
of 23 morphological and ecological characters. After the exclusion ofM. williamsoni,
known from only a single male, and inclusion of subspecies within their nominate
taxa, a single resolved tree of length 85 was obtained; this tree has a consistency
index of 0.56 and a retention index of 0.72. Based on this phylogeny, it appears
that the major clades within Megalagrion differentiated on Kaua'i or an antecedent
high island. These clades subsequently colonized the younger islands in the chain
in an independent and sequential fashion. The phylogeny also implies an ecological
progression from ancestral breeding sites in ponds or slow stream pools to breeding
on seeps, with the latter habitat having given rise on one hand to a clade of species
breeding in phytotelmata or terrestrially, and on the other hand to a clade breeding
in rushing midstream waters. The latter ecological progression also indicates a
transformation series in larval gill structure from foliate to saccate and eventually
to lanceolate. Most species of current conservation concern are shown to be clustered
in particular clades, indicating an inherent phylogenetic vulnerability of certain
taxon clusters to novel ecological perturbations; the additional species at risk not
present in the above clades are endemics confined to the island of O'ahu and have
declined because of their geographic provenance.